Shale gas operations triggered Blackpool tremors

Deep-level drilling caused two minor earthquakes

Shale consortium Cuadrilla says deep-level drilling caused two minor earthquakes in Lancashire, but they were so small hardly anyone felt them. The group's report, a Geomechanical Study of Bowland Shale Seismicity, which coughed to the tiny tremors, was published on Wednesday.

Cuadrilla says a quake of magnitude 2.3 on 1 April, and one of 1.5 on 27 May, were probably caused by fracking - or hydraulic fracturing. The group said:

In the Lancashire area of the UK, there have been many mining-induced seismic events at comparatively shallow depths of around one kilometre below the surface, that measured up to 3 on the Richter scale. The Preese Hall-1 well is more than 1.5 kilometres below the earth’s surface so events of similar strength are very unlikely to cause any damage at the surface.

Fracking takes place for a short but intense period, after which the heavy machinery departs and the "pad", or well head, collects the gas released.

Neither the quakes, nor their intensity, come as much of a surprise. Coal excavation causes hundreds of minor earthquakes around the globe every year - but shale 'fracking' typically takes place at a much deeper level: several thousand metres below most quake-inducing coal seams. The Lancashire shale bed lies 7,000 to 12,000 feet below the surface.

Cuadrilla attributes the seismic activity to the repeated injection of the fracking fluid - water with a trace of lubricant - into a pre-existing critically stressed fault, but says this is unlikely to reoccur. Still, Cuadrilla suspended drilling as a precaution.

All the same, the group accepts the conservative German standard for safe ground vibration, which sets the maximum magnitude at 2.6 and recommends mitigation measures if seismic activity exceeds magnitude 1.7 - after which the group should stop water injection and reduce pressure.